In most optical data transmission applications using optical wavelength division multiplexing (WDM), the wavelengths of the optical transmitters (mostly lasers) need to be locked to dedicated channel wavelengths, for example according to the ITU wavelength grid with optical frequencies spaced by 100, 50, or 25 GHz. For this purpose, wavelength lockers are available using optical filter devices, like etalons, which are periodic with the ITU frequency spacing. The known methods for locking the wavelength or optical frequency of a transmit signal to a dedicated channel frequency use a fixed relative position of the periodical optical transfer function of an optical filter device in order to lock the frequency of the transmitter unit to a fixed position of the respective transmission band of the filter. This fixed position usually corresponds to a mid point between the minimum and the maximum point of the filter transfer characteristic.
However, in some applications, optical wavelengths or optical frequencies for the channel transmit signals are required, which do not match the ITU grid. An example of such an application is a data transmission system using a cyclic arrayed waveguide grating (cyclic AWG) for demultiplexing an optical WDM signal comprising optical channel signals lying in one optical band (for example the L-band) and multiplexing channel transmit signals guided in the opposite transmission direction and lying in another optical band (for example the C-band). In this case, the cyclic AWG may be used for demultiplexing the WDM receive signal and multiplexing the channel transmit signals into a WDM transmit signal, if the channel receive signals and the channel transmit signals are spaced by a multiple integer of the free spectral range (FSR) of the cyclic AWG, that is, in different refractive orders of the cyclic AWG. As, due to physical properties of a cyclic AWG, the frequency spacing in different, even neighboring orders is slightly different, the frequency spacing of the downstream and upstream channels is different, too. In such applications, the intrinsic frequency spacing of the channel signals for the upstream or downstream direction differs from the ITU spacing by a few GHz, even if the channel signals for the respective other direction match the ITU spacing. Another example, where channel frequencies are required which do not fit the ITU grid, is the choice of unequal channel spacing to combat the non-linear effect of four-wave mixing. Here, three optical signals in a WDM system at frequencies f1, f2, and f3 interact during transmission in a non-linear fiber, generating new signals at, for instance, f1+f2−f3. If the frequencies adhere to a fixed frequency grid, the generated new signal is likely to interfere with a further signal of the WDM system. Using frequencies in the WDM system which do not have a fixed frequency spacing reduces the distortion from four-wave mixing, as most mixing products fall between the data carrying channels. This application is known as unequal channel spacing. For this application, it is necessary to control the transmit lasers to arbitrary frequencies.
Unfortunately, wavelength-locking devices for arbitrary but fixed absolute frequencies and arbitrary but fixed frequency spacings are not readily commercially available, or they are prohibitively costly as it is necessary to manufacture optical filter devices having a periodicity matching the required frequency spacing in rather low numbers.
Of course, it would be possible to use optical filter devices having a periodical transfer function, which are tunable with respect to the absolute position of the transmission bands (or peaks) and with respect to the spacing of the bands (or peaks), like a tunable delay-line interferometer. However, in case of a tunable delay-line interferometer, the absolute frequency and the frequency spacing of the transmission bands (or peaks) strongly varies with temperature, so the temperature of such a device needs to be precisely controlled during operation. Alternatively, a control signal having a fixed wavelength or optical frequency may be used to control the frequency grid of the interferometer. However, both variants require additional effort and costs.
It is therefore an object of the present invention to provide a frequency locking method for tuning each of a plurality of narrow-band optical channel transmit signals to a dedicated optical channel frequency, the channel transmit signals having arbitrary channel frequency spacings, which allows the use of readily available optical filter devices having a transfer function with a different periodicity. It is a further object of the invention to provide a wavelength-locking device adapted to realize the method according to the invention.